Recently, small-sized mobile terminals represented by, for example, smartphones and tablets, have rapidly become widespread. Therefore, a demand for small-sized and high energy density batteries that drive the small-sized mobile terminals, has increased.
Typically, a graphite-based material is used for a negative electrode of a lithium ion battery. Theoretical capacity of the graphite-based material is 372 mAh/g (LiC6). Currently, the theoretical capacity has nearly approached its limit. Development of a new electrode material is required in order to further improve energy density. In particular, as examples of a negative electrode material, materials to be alloyed with lithium, such as silicon and tin, with low potential and a small electrochemical equivalent, following carbon and lithium, have attracted attention.
In particular, lithium can be inserted into silicon by a ratio of 4.4 atoms of lithium to one atom of silicon. Theoretically, silicon can have capacity approximately 10 times that of a graphite-based carbon material. However, when lithium is inserted into a silicon particle, the volume of the silicon particle expands approximately three to four times. Thus, there is a problem that the particle itself cracks due to repeats of charge and discharge so as to pulverize, or so as to influence other members included in an electrode.
Micronization of the size of a silicon particle is effective in inhibiting the pulverization. However, when the size of the particle decreases, aggregation also easily occurs. Therefore, a countermeasure, such as covering of a silicon particle with silicon oxide or a carbonaceous substance, has been made. However, currently, capacity degradation has not been sufficiently inhibited.